1. Field
The present disclosure generally relates to composite structures and, in particular, to the fabrication of composite structures. Still more particularly, the present disclosure relates to a method and apparatus for producing composite fillers used to fill gaps in composite structures.
2. Background
Composite materials are tough, lightweight materials created by combining two or more functional components. For example, a composite material may include reinforcing fibers bound in a polymer resin matrix. The fibers may be unidirectional or may take the form of a woven cloth or fabric. In thermoset composites, fibers and resins are arranged and cured to form a composite material.
When composite structural members are joined together, gaps or voids may be present along bond lines between the members which may need to be filled in order to increase the strength of the bond. For example, in the aircraft industry, composite fuselage stiffeners such as stringers may include adhesive filler at the radius bond line between the stringer and a fuselage skin. The adhesive filler is applied in the form of triangular cross section strips, sometimes referred to as noodles or fillers, which fill the voids at the bond line. The adhesive filler may be formed from composite materials such as adhesive, prepreg tape, or fabric.
When a stiffener possesses sufficient pull-off strength in the area of the filler, the stiffener resists tension loads imposed on the stiffener for a given application. In order to achieve adequate pull-off strength, it may be necessary to increase the gauge of the stiffener, thereby adding weight to the aircraft. Alternatively, radius blocks may be added to the stiffeners in order to increase pull-off strength, but the radius blocks may add undesirable weight, complexity, or cost to the aircraft.
The filler may be formed by extruded material, pultruded material, rolled composite material, or stacked strips of composite material parallel to the aircraft skin. When the filler is of sufficient stiffness, the filler may transfer some of the load from the stiffener into the base. If the filler is not sufficiently strong it may not function to transfer load.
Using pultruded material in combination with laminate material may produce undesirable results. For example, cracks may be observed between a pultruded material tip and laminate stack in a composite filler. Further, the coefficient of thermal expansion of pultruded material may be different than the coefficient of thermal expansion of a laminate material. This difference in the coefficients of thermal expansion may result in internal stresses within the composite filler.
Therefore, it would be desirable to have a method and apparatus that take into account at least some of the issues discussed above, as well as other possible issues. Specifically, it would be desirable to have a method and apparatus that would reduce cracking.